An arrangement is known from Japanese Patent Application Laid-open No. 2001-27118 wherein, with regard to a Rankine cycle system in which a turbine is driven by heating, with exhaust gas, steam of a coolant for cooling an internal combustion engine, the amount of heat dissipated (amount of heat transferred) from the periphery of a combustion chamber of the internal combustion engine to the coolant is controlled in accordance with- the running conditions of the internal combustion engine so as to optimize the steam temperature at an inlet of the turbine, thereby improving the efficiency.
When the temperature of steam generated by an evaporator of the Rankine cycle system is set low relative to the temperature of the exhaust gas supplied to the evaporator, the difference in temperature between the exhaust gas and the steam increases, the heat exchange efficiency of the evaporator increases, the temperature of the exhaust gas discharged from the evaporator decreases, and the energy of steam thus generated increases (see FIG. 24A). When the temperature of the steam generated by the evaporator is set high relative to the temperature of the exhaust gas supplied to the evaporator, the difference in temperature between the exhaust gas and the steam decreases, the heat exchange efficiency of the evaporator decreases, the temperature of the exhaust gas discharged from the evaporator increases, and the energy of the steam thus generated decreases (see FIG. 24B). The higher the temperature of the steam supplied from the evaporator, the higher the efficiency of an expander, which is operated by the steam.
That is, as shown in FIG. 24A, when the temperature of the steam generated by the evaporator is set low, the efficiency of the evaporator is high, but the efficiency of the expander is low, and as shown in FIG. 24B, when the temperature of the steam generated by the evaporator is set high, although the efficiency of the evaporator is low, the efficiency of the expander is high. There is therefore a steam temperature at which the overall efficiency, which takes into consideration the efficiency of the evaporator and the efficiency of the expander, becomes a maximum. Moreover, the efficiency of a displacement type expander depends not only on the steam temperature, but also on a steam pressure at which the efficiency, which depends on the expansion ratio and the leakage of the expander, becomes a maximum.